1. Field of the Invention
The invention relates to heating mechanisms for process chambers, particularly chemical vapor deposition chambers.
2. Description of the Related Art
Chemical vapor deposition (CVD) is a popular process for depositing various types of films on substrates and is used extensively in the manufacture of semiconductor integrated circuits. In CVD processing, chemicals containing atoms required in the final film are mixed and reacted in a deposition chamber. The elements or molecules deposit on the substrate surface and build up to form a film. The substrate upon which the film is to be deposited is usually mounted on a susceptor which, depending on the type of CVD process, can be comprised of a variety of materials. A susceptor preferably will have good thermal conductivity and a high resistance to thermal deformation. Aluminum, for example, is a popular susceptor material with good thermal conductivity, but which is too fragile to withstand high temperatures. Hence, susceptors made of glass carbon or graphite coated with aluminum nitride (AlN) have become popular.
There are two basic types of heating schemes used in CVD systems which are distinguished based upon how the susceptor is heated: resistive heating schemes utilize a resistive heating element to directly heat the wafer and produce a reaction which is more localized at the wafer. Lamp heating schemes use a radiant heating lamp which heats the substrate, susceptor and chamber and produces a reaction which is present throughout the chamber.
In a lamp heated system, the substrate is carried on a susceptor and heat is transferred to the substrate by the lamps positioned behind heat resistant protective glass in the chamber. In a resistive heating system, resistive heating elements are located in the wafer holder. In general, resistive heating systems allow more accurate control of the process occurring in the chamber. In lamp heating systems, for example, the heat resistant glass behind which the radiant heaters are positioned will itself become coated with the materials used in the process chamber. The products of the process will deposit on the glass, reducing the effectiveness of the lamp heaters and corroding the glass after repeated processing.
Further, in CVD systems, a thermocouple is connected to the substrate holder to measure thermal variations of the holder under processing. In a lamp heated system, the thermocouple must be in the chamber. In a resistive heating system, the thermocouple can be placed in a controlled environment within the stem holding the heating element. This adds to the life of the thermocouple and increases the accuracy of the thermocouple since it is not exposed to the elements in the processing chamber.
Still further, the serviceability of lamp heated CVD chambers is more of a concern than with resistive heating systems. For example, because susceptors are required to carry the substrates into the process chamber in a lamp heating system, calibration between the mechanical components which transfer the susceptor and wafers between chambers can be problematic.
An exemplary CVD process which is useful for a variety of semiconductor applications is the dichlorosilane (DCS) tungsten silicide process. Because of the temperatures at which the DCS process occurs, conventional resistive heating systems are generally not suitable for the process as they are not capable of sustaining the required process temperature range (500.degree.-600.degree. C.). As a result, the process is performed in, for example, a halogen lamp heated CVD chamber. However, providing a resistive heating chamber in which a DCS process could take place would be advantageous.
In a DCS tungsten silicide process, the tungsten silicide film is formed by a reaction of WF.sub.6, DCS, and SiH.sub.4. As with other CVD processes, after processing a series of wafers, typically twenty-five, the chamber is cleaned to remove products of the reaction which have deposited on the walls of the reaction chamber and other components inside the chamber. During the cleaning process, the wafer holders remain in the CVD chamber.
Two different types of cleaning processes are generally utilized: chemical cleaning or plasma cleaning. Plasma cleaning involves generating a plasma using NF.sub.3 and RF energy. As a result, plasma cleaning is more localized, but results in an uneven cleaning of the deposits since it is more difficult to control. If the plasma clean process is performed at temperatures in the 500.degree.-600.degree. C. range, the susceptor will be severely damaged and a great deal of particulate matter will be generated from the other components of the system. In addition, plasma cleanings are localized and less uniform. Chemical cleaning is more uniform, but is more stressful on the components of the chamber.
Chemical cleaning involves placing ClF.sub.3 in the process chamber to produce a thermally dependant reaction which is more severe at higher temperatures. Chemical cleaning can damage the susceptor if not properly controlled. Chemical cleaning in chlorine tetrafluoride (ClF.sub.3) at temperatures of 300.degree. C.-600.degree. C. is undesirable as it can cause both mechanical and chemical stresses on susceptors. For example, when using a glass carbon susceptor, the chemical cleaning must be conducted at a temperature of 200.degree. C. This requires cooling the process chamber from the 500-600.degree. C. DCS tungsten silicide processing temperature, thereby slowing the processing throughput of the chamber.
To date, radiant heating CVD systems have been preferable for use in processes like the DCS tungsten silicide process. Such systems are more resistant to heat stress and the chemical cleaning processes utilized in such process. However, it would be desirable to provide a resistive heating CVD chamber which can be utilized in the CVD process. This should bring to bear all the advantages of the cold wall resistive heating systems in the DCS process.